1. Field of the Invention
The present invention relates to a laser beam machining method and a laser beam machining apparatus by which a transparent substrate of a wafer, having a functional layer at a surface of the transparent substrate and having devices formed in a plurality of regions demarcated by a plurality of streets formed in a grid pattern on the surface, is machined through ablation by irradiation with a laser beam.
2. Description of the Related Art
In a semiconductor device manufacturing process, a plurality of regions are demarcated by planned dividing lines called streets which are arranged in a grid pattern on a surface of a semiconductor wafer having a substantially circular plate-like shape, and devices such as ICs and LSIs are formed in the thus demarcated regions. Then, the semiconductor wafer is cut along the streets to divide the regions with the devices formed therein, whereby individual semiconductor chips are manufactured. In addition, an optical device wafer in which light emitting devices or the like such as light emitting diodes (LEDs) are layered on a surface of a sapphire substrate is also cut along streets into individual optical devices such as light emitting diodes, which are widely used in electronic apparatuses.
The cutting of the wafer along the streets as above-mentioned is ordinarily carried out by use of a cutting apparatus called dicer. The cutting apparatus includes a chuck table for holding a work such as the wafer, cutting means for cutting the work held on the chuck table, and cutting feeding means for effecting relative movement of the chuck table and the cutting means. The cutting means includes a cutting tool including a rotary spindle and a grinding blade attached to the spindle, and a driving mechanism by which the rotary spindle is driven to rotate. In such a cutting apparatus, while the cutting tool is being rotated at a rotating speed of about 20,000 to 40,000 rpm, the cutting tool and the work held on the chuck table are put into relative cutting feed. The cutting by such a cutting device, however, is disadvantageous in that the machining speed cannot be enhanced in some cases depending on the kind of the wafer and, hence, the cutting is not necessarily satisfactory in regard of productivity.
On the other hand, in recent years, as a method for dividing a wafer, in which optical devices including a nitride semiconductor or the like are layered on a surface of a sapphire substrate, along streets, there has been proposed a method in which the wafer is irradiated with a pulsed laser beam along the streets formed on the wafer to form laser beam-machined grooves and an external force is exerted on the wafer along the streets to break up the wafer along the streets (see, for example, Japanese Patent Laid-Open No. Hei 10-305420). According to the laser beam machining method described in the patent document, the laser beam-machined grooves can be formed at a comparatively high machining speed.
In the case of the laser beam machining method described in Japanese Patent Laid-Open No. Hei 10-305420, however, wall surfaces of the laser beam-machined grooves formed along the streets have been melted and are rough, which causes a problem that where the individual devices divided up from the wafer are light emitting diodes (LEDs), the LEDs exhibit lowered luminance. For overcoming this problem, a machining method has been proposed in which the machined grooves are formed in the shape of broken lines (see, for example, Japanese Patent Laid-Open No. 2007-149820). According to this laid-open patent document, the machined grooves are formed in the shape of broken lines, whereby the once-melted layers at side surfaces of the devices can be reduced in area, and the lowering in luminance can be suppressed to a slight extent.
In the case of the laser beam machining method described in Japanese Patent Laid-Open No. 2007-149820, however, an unmachined area where no machined groove is formed (omission of the machined groove) may be generated in the portions irradiated with the laser beam for the purpose of forming the machined grooves. FIG. 11 is an electron microphotograph showing a situation in which omission of the machined groove as just-mentioned has been generated. Generation of the unmachined area where no machined groove is formed would not only lower the dividability of the wafer but also cause damage to the device layer through a process in which the laser beam not absorbed into the sapphire substrate is radiated to the device layer.